Polyurethane adhesives are utilized to bond various types of substrates together, for example, glass, plastic, fiber reinforced plastic and metals which may be coated or uncoated, see U.S. Pat. No. 5,719,252 and U.S. Pat. No. 5,143,996. In the automobile industry two part polyurethane adhesives are used to bond tail gates, roof modules, spoilers and trim parts. It is desirable that the stiffness of the adhesive after cure remains relatively constant across the use temperature of the structure bonded together. Stiffness of a polymeric material can be measured by determining the G-modulus, which is measured according to DIN EN 6721-2. For automobiles such use temperature ranges from about −30° C. to about 100° C. over long term exposures and for short term exposure up to about 180° C. Many cured polyurethane adhesive compositions exhibit a glass transition in such temperature range. When a material goes through a glass transition the stiffness and G-modulus are negatively affected. In certain applications any reduction of stiffness negatively impacts the ability of the assembled structure to remain bonded.
In the assembly of parts in industry time is crucial and the curing speed of adhesives is an important parameter. Further, for many substrates a primer is required for an adhesive to bond to a surface which adds steps and cost to the assembly of structures. Thus, adhesives that can bond to certain substrates without the need for a primer are desired.
What is needed is a two part polyurethane which exhibits a low change in stiffness, G-modulus, over the anticipated use temperature which cures rapidly, and bonds well to substrates without the need for a primer.